1. Field of the Invention
The present invention relates to a semiconductor memory device and a data erase method thereof, and is applied, for example, to a NAND flash memory.
2. Description of the Related Art
In recent years, semiconductor memory devices, for example, NAND flash memories, have begun to be used as main memories of various electronic devices, such as portable audio devices, by virtue of their merits of large capacity and nonvolatility (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2005-235260).
Under the circumstances, in addition to the enhancement in functions, the increase in memory capacity is a challenge to the NAND flash memory. The simplest method for realizing the increase in memory capacity is to promote microfabrication of memory cells. However, if the microfabrication of memory cells is advanced, the distance between a word line (WL) and a select gate (SGS, SGD) decreases.
As a result, an electric field between the word line (WL) and the select gate (SGS, SGD) increases, and the defect ratio of memory cells increases due to hot carriers which are excessively injected in floating electrodes (FG) of memory cells which neighbor select transistors (S1, S2). In particular, there arises such a problem that the defect ratio of memory cells neighboring the source line side select transistor (S1) increases.
On the other hand, it may be thought that the hot carriers, which are injected in the floating electrode, may be released into the semiconductor substrate by an erase operation. However, in order to release the hot carriers into the semiconductor substrate, it is necessary to make the well voltage higher, and to increase the gate voltage of the select transistor (S1, S2) by capacitive coupling with the well. If the gate voltage of the select transistor (S1, S2) is increased, however, the select transistor (S1, S2) is damaged by gate dielectric breakdown. Thus, from the standpoint of gate breakdown voltage of the select transistor (S1, S2), there is a limitation to the release of hot carriers into the semiconductor substrate.